To improve diagnostic abilities in patients with insulin-producing tumors by ex- amining plasma insulin concentration in states where insulin secretion is normal ly suppressed due to a falling blood sugar. Plasma insulin secretion will be ex- amined using measurement of related proteins which are secreted with insulin from the same cell. These are called C-peptide and proinsulin. There are several advantages to the measurement of these proteins, including the possibility that diagnosing insulin-secreting tumors may be simplified.